It's election day on much of the South Shore, a conservative-leaning part of the state where Republicans hope to hold on to one of their few Senate seats.

Weymouth, Hingham, Cohasset, Scituate, Norwell, Hull and Marshfield voters are at the polls to pick a new State Senator to replace Bob Hedlund, the Weymouth Republican who gave up his seat to become one of only seven GOP mayors in the state. Hedlund's exit prompted the May Special election.

Democrat Joan Meschino, an attorney, CEO and former Hull selectman, faces Republican, and former Hedlund staffer, Patrick O'Connor, the president of Weymouth's Town Council. The stretch of coastal communities from Weymouth down to Duxbury is one of the friendliest parts of the state to the GOP. O'Connor has had great success on a funding level, doubling Meschino's total campaign warchest, and even has the support of several labor unions, including the AFL-CIO.

Two other races for the Legislature - a House seat in Lynn, as well as a Senate seat in East Boston and just north of the city - will be won by Democrats today since no Republicans are running.

But today may not be the end of the fight for Hedlund's Senate seat. Both O'Connor and Meschino already have enough signatures for a rematch on the November ballot when droves of voters come out to vote in the presidential election. That is, if the loser of today's race manages to win their party primary and face off against the incumbent.

"It'll be a much bigger electorate, there will be very short of time for whoever is in the seat to be able to create a  record and prove their capacity, but it's going to be a huge turnout election," Senate President Stan Rosenberg, a supporter of Democrat Meschino, told reporters Tuesday.

The South Shore race is one of the few active competitions between competitive Republicans and Democrats, but two primary election fights worth keeping an eye on are shaping up in Cambridge and Somerville and on  Cape cod.

Veteran Sen. Pat Jehlen (D-Somerville) faces a challenge for her seat from Cambridge City Councilor Leland Cheung. Cheung, who ran unsuccessfully for Lt. Governor in 2014, is pressuring Jehlen to disclose campaign donors monthly, far more frequently than that's required of legislative candidates under the law.

“The facts are that since first running for State Senate 11 years ago, Senator Jehlen has raised more than a third of her itemized contributions from PACs and lobbyists and another $50,000 in unitemized contributions from donors whose names aren’t publically available. It’s unacceptable that we don’t know who those donors are," Cheung wrote in a statement bashing Jehlen.

Jehlen is a progressive favorite from the very progressive reaches of Camberville, and with five terms under her belt, will be tough to out- campaign at the grassroots level. Cheung is well-funded, but has an uphill climb ahead of him to oust an intrenched incumbent.

On the Cape, Sen. Dan Wolf's (D-Harwich) decision not to seek a third term opened the field for several Democrats to jump into the fray to succeed him, including Rep. Brian Mannal (D-Barnstable,) former staffer at the Department of Mental Health Julian Cyr and current Barnstable County Commissioner Sheila Lyons.